1. Field of the Invention
This invention relates to the field of Synchronous Optical Network (SONET) technology. In particular, the invention relates to increasing the efficiency of SONET communications by minimizing the response time for the performance of protection switching.
2. Description of the Related Art
Synchronous Optical Networks (SONET) are fiber-optic transmission systems for high-speed digital traffic. The SONET technology is an intelligent system that provides advanced network management and a standard optical interface. The SONET technology is typically deployed in SONET rings that originate and terminate in a carrier's facility. FIG. 1 is a typical simple SONET ring 100. While SONET networks or rings can be quite complex, the typical SONET network 100 includes numerous network elements 102-112 linked by upstream and downstream communication fibers 120. A ring 100 may span only a few miles or stretch thousands of miles, depending on its purpose. The network elements available for use in SONET networks 100 include terminal multiplexers (TMs), add-drop multiplexers (ADMs), digital loop carriers (DLCs), digital cross-connects (DCSs), matched nodes (MNs), and drop and repeat nodes (D+R).
The network elements typically used in SONET rings 100 include ADMs and DCSs. An ADM is a device that connects individual lines to backbone trunks and multiplexes lower-speed electrical and/or optical signals into a high-speed optical channel and vice versa. A DCS is a network device used to switch and multiplex low-speed voice and data signals onto high-speed lines and vice versa.
The SONET rings are self-healing in that they use two or more transmission paths among network nodes and, if there is a break in one path or a loss of signal in one path, network traffic may be rerouted around the damaged path. This path protection switching provides alternate signal paths among network nodes and is a major advantage in the deployment of rings. For the best security against failure, when possible, different physical routes are used for the two transmission paths. The SONET technology typically supports levels of service including optical carrier (OC) and synchronous transport signal (STS). Optical carrier service refers to optical signals, and synchronous transport signal refers to electrical signals.
In operation, a switching event is generated in a SONET ring in response to a network condition that threatens service quality. When a switching event occurs in a SONET ring, the event is detected and the information necessary to provide alternate signal paths using protection switching is propagated among the network elements of the ring. Each network element along the propagation path detects the switching event and propagates the protection switching information to the next element in the ring. The switching event information is transferred through the network using two bytes of the SONET frame, referred to as the K1 and K2 bytes.
A problem arises in SONET ring protection switching in that the time allowed for propagation of switching event information among network elements is limited to a total of 50 milliseconds. Typically, numerous switching events can be occurring simultaneously in a number of network elements as a result of a single switching event propagating along the network and adversely affecting many network elements. As such, the propagation time problem is compounded. Therefore, as the number of network elements in a ring increases, the propagation time allowed between each element decreases. For example, if a ring contains 16 nodes, the propagation time is reduced to approximately 3 milliseconds between each node.
The switching event detection and propagation problem is further compounded by the control structure of a typical SONET ring. The control structure is provided in order to control network elements in providing alternate signal paths during path switching initiated by a switching event. A typical control structure couples a number of network elements to a central processor hub or card via Ethernet connections. Information regarding switching events, when propagated, is detected by network elements along the propagation path. The network elements transfer the detected information to the processor hub using the Ethernet coupling. The processor hub, while communicating with and receiving switching event information from a number of network elements, processes the information and initiates appropriate action via communications with the affected network elements.
The problem arises in that, as numerous network elements are typically receiving the switching event information at about the same time, a number of network elements are communicating with the processor hub regarding the switching event information at about the same time. This leads to information collisions in the processor hub. When an information collision occurs, the information or data involved in the collision is returned to the source network element where it is held for at least some prespecified period of time before being recommunicated. For example, some systems may hold returned data for approximately 5 milliseconds before attempting recommunication. As a result of these information collisions, the propagation time limits are often violated in typical rings.
This collision problem might be reduced using switched mode Ethernet couplings. However, the switched mode is much more expensive and space consuming because it requires more processing power and additional space to implement the switch logic. As there is a need to reduce the expense and physical size of the SONET equipment, the switched mode would not provide a viable solution.
Another solution to this problem might be found using bus connections among the nodes. This, too, is problematic in that the bus requires more interconnection among nodes and results in an architecture where a single point failure is fatal. As such, a bus fault would result in a system failure. While a dual redundant bus might prevent the single point of failure, it requires significantly more interconnection among nodes and introduces a complex control system. Consequently, a method and apparatus is needed that minimizes the response time in the performance of protection switching in SONET networks or rings.